What is the most computationally efficient way to make ripples?
The most computationally efficient (and easiest) way to make ripples or waves in Blender is to use the Wave Modifier.

This feature was in Blender long before fluid simulations, dynamic paint, displacement textures or even modifiers. (It was called an "Effect" back then, but other than that, it has not changed much.)  



I just tested it on Blender 1.60 from 1999. I bet it was in Blender sooner, but I don't have an Irix machine to test it.  (I also know a similar wave tool was in 3ds Max r2 in 1997.)

It's a simple procedural operation designed on much slower computers, so it's fast. (I believe it uses the sine function to offset the mesh, also it doesn't need to cache.)  You can interactively tinker with the settings and preview the animation.

It has options for the size of the wave, when and where it starts, whether it's a ripple circle or a wide wave...  

It is simple, but you can do more advanced effects with it.  For example: to emulate a ripple bouncing off a wall, add another wave with the starting position outside of the object, the same distance to the wall. 



Here's a video tutorial on using the wave modifier by the Ott Planetarium.